The present invention relates to a semiconductor device, particularly to an art to be effectively applied to a semiconductor device comprising a nonvolatile memory device including an insulator film formed between a floating gate electrode and a control gate electrode. The present invention is especially directed to a nonvolatile memory (including, e.g., stacked type memories; or split type memories; or stacked type memories optionally having an erase gate in addition to the control and floating gates; or split type memories optionally having an erase gate in addition to the control and floating gates, with insulator films between the various gates), such as a flash memory.
A semiconductor device uses a nonvolatile memory device referred to as a flash memory. Because the flash memory is superior in portability and impact resistance and electrically allows on-board bulk erasing, it is anticipated as a file memory of a future compact portable data unit.
The flash memory is provided with a memory cell array section constituted by arranging a plurality of memory cells using a nonvolatile memory device as a memory unit like a matrix. The nonvolatile memory device is constituted on the surface of a semiconductor substrate made of, e.g., single crystalline silicon.
The above nonvolatile memory device mainly comprises a semiconductor substrate serving as a channel region, a first gate dielectric film, a floating gate electrode, a second gate dielectric film, a control gate electrode, and a pair of semiconductor regions serving as a source region and a drain region (also referred to as impurity diffusion layers). The nonvolatile memory device injects electrons into the floating gate electrode of the semiconductor substrate by applying a positive voltage to the control gate electrode of the semiconductor substrate, and stores one-bit data (“0” or “1”) in accordance with the difference in the threshold voltages of memory cell transistors. Moreover, the first gate dielectric film denotes a tunnel dielectric film formed between the semiconductor substrate and the floating gate electrode. Furthermore, the second gate dielectric film denotes, e.g., an interpoly dielectric film formed between the floating gate electrode and the control gate electrode.
In the case of the nonvolatile memory device, the floating gate electrode and the control gate electrode are each respectively formed from a polycrystalline silicon film, and the first gate dielectric film and the second gate dielectric film are respectively formed from a silicon oxide (SiO2) film. A silicon oxide film serving as the first gate dielectric film is formed by applying thermal oxidation to the surface of a semiconductor substrate made of single crystalline silicon, and a silicon oxide film serving as the second gate dielectric film is formed by applying thermal oxidation to the surface of a floating gate electrode made of a polycrystalline silicon film.
The silicon oxide film formed on the surface of the floating gate electrode made of the polycrystalline silicon film has a low breakdown voltage and is inferior in retention capability compared to a silicon oxide film formed on the surface of a semiconductor substrate made of single-crystal silicon. Therefore, in the case of flash memories of 4 [Mbit] onward, as the second gate dielectric film there is formed, in place of the single-layer silicon oxide film, a composite film, so-called an ONO (Oxide/Nitride/Oxide) film, obtained by superimposing a silicon oxide film, a silicon nitride (Si3N4) film, and a silicon oxide film in order on the floating gate electrode. This is because, when film thicknesses in terms of a silicon oxide film are the same, an ONO film has a small leakage current compared with a silicon oxide film. This art is discussed in “IEEE Transaction on Electron Devices, 38 (1991) pp. 386–391”.